Don't use Mailbox::SetName in drawing buffer tests

There is no real reason for it, we can just assign mailbox.

Change-Id: I473adab13adbf2c2bccf1c92f29cbde9a3f128b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6646928
Reviewed-by: Saifuddin Hitawala <hitawala@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1474355}